Five dogs with pre-existing multivessel coronary occlusive disease underwent three acute balloon occlusions of the left anterior descending coronary artery. Nitroglycerin, nitroprusside, and no therapy were given in random order to test the relative ability of these vasodilator agents to nodify the extent of ischemic injury noted under control conditions. Despite inducing nearly identical decreases in arterial pressure and increases in heart rate, nitroprusside was found to increase the degree of ischemic injury, while nitroglycerin caused a decrease in ischemic injury.